Abstract-Ultrafast Electron-Lattice Coupling Dynamics in VO2 and V2O3 Thin Films

Elsa Abreu, Stephanie N. Gilbert Corder, Sun Jin Yun, Siming Wang, Juan Gabriel Ramirez, Kevin West, Jingdi Zhang, Salinporn Kittiwatanakul, Ivan K. Schuller, Jiwei Lu, Stuart A. Wolf, Hyun-Tak Kim, Mengkun Liu, Richard D. Averitt

(Submitted on 19 Jan 2017)

https://arxiv.org/abs/1701.05531

Ultrafast optical pump - optical probe and optical pump - terahertz probe spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the electronic oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in the metallic state of both materials. The low energy Drude response of V2O3 appears more susceptible than VO2 to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold increase in V2O3 across the phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of VO2, and suggest that a similar phenomenon might be at play in the metallic phase of V2O3. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, opening the way to similar studies in other complex materials.

Abstract-Terahertz Coded Aperture Mask using a Vanadium Dioxide Bowtie Antenna Array

Souheil Nadri, Rebecca Percy, Lin Kittiwatanakul, Alex Arsenovic, Jiwei Lu, Stu Wolf, Robert M. Weikle II

http://arxiv.org/abs/1512.02697

Terahertz imaging systems have received substantial attention from the scientific community for their use in astronomy, spectroscopy, plasma diagnostics and security. One approach to designing such systems is to use focal plane arrays. Although the principle of these systems is straightforward, realizing practical architectures has proven deceptively difficult. A different approach to imaging consists of spatially encoding the incoming flux of electromagnetic energy prior to detection using a reconfigurable mask. This technique is referred to as coded aperture or Hadamard imaging. This paper details the design, fabrication and testing of a prototype coded aperture mask operating at WR 1.5 (500 to 750 GHz) that uses the switching properties of vanadium dioxide (VO2). The reconfigurable mask consists of bowtie antennas with vanadium dioxide VO2 elements at the feed points. From the symmetry, a unit cell of the array can be represented by an equivalent waveguide whose dimensions limit the maximum operating frequency. In this design, the cutoff frequency of the unit cell is 640 GHz. The VO2 devices are grown using reactive-biased target ion beam deposition. A reflection coefficient (S11) measurement of the mask in the WR 1.5 (500 to 750 GHz) band is conducted. The results are compared with circuit models and found to be in good agreement. A simulation of the transmission response of the mask is conducted and shows a transmission modulation of up to 28 dB. This project is a first step towards the development of a full coded aperture imaging system operating at WR 1.5 with VO2 as the mask switching element.